Access modifier (C # programming guide)

Public

Any other code in the same assembly or other assembly that references the Assembly can access this type or member.

Private

Only the code in the same class or structure can access this type or member.

Protected

Only code in the same class or structure or derived class can access this type or member.

Internal

Any code in the same assembly can access this type or member, but the code in other assembly cannot.

Protected internal

Any code in the same assembly or any derived class in other assembly can access this type or member.

Http://msdn.microsoft.com/zh-cn/library/ms173121.aspx